Interaction of groundwater and shallow lakes on outwash sediments in the sub-humid Boreal Plains of Canada

The hydrologic controls on the interaction between a shallow lake and a groundwater flow system have been determined for a coarse-textured landscape on the Boreal Plains of Canada. Lake-groundwater interactions were studied to provide an understanding of the near-surface hydrologic processes in a sub-humid climate, where annual precipitation is equal to, or less than potential evapotranspiration. This study was initiated to assess potential landscape impact from forestry and energy industrial activity, and future scenarios of climate change expected in the Western Boreal Forest. A 39 ha lake on outwash sediments was studied for two consecutive years. Hydrometric measurements and stable isotopic analyses indicated that evaporation was the dominant hydrologic flux during ice-free months, and was primarily responsible for a 0.2 m decline in lake level during this study. A combination of summer rain and consistent supply of groundwater discharge throughout each year were the dominant sources of water. The near equivalence of precipitation and evaporation, and relative dominance of each of these processes during the hydrologic year, resulted in the formation of a water table mound downgradient of the lake, and its disappearance each year. The presence of these humid (mound) and arid (complete disappearance) phenomena at one location within the same year, reveal the sensitivity of hydrologic systems in this sub-humid region to small variations in climate. The onset and duration of evaporation from the lake surface was a major controlling factor on groundwater exchange, to such a degree that the shallow lakes are thought to be evaporation windows in this landscape, creating sub-surface capture zones that depend on lake size and landscape position. These findings indicate a very dynamic relationship between atmospheric water fluxes and groundwater interaction, which could potentially conceal anthropogenic impacts within natural variability.